Gas sensors have been in use for several decades. The principle of most current gas sensors relies on chemical interactions between the gas and a specific material which provides high sensitivity. It is well known that such chemical sensors suffer from poor stability over time, due to involving chemical reactions. In order to circumvent this problem thermal conduction sensors that make only use of the physical properties of gases, have been developed.
Thermal conductivity measurement has been used in gas chromatography for more than 100 years, this gas detection method is still of large importance in process control and gas analysis. Furthermore, it presently experiences a revival due to the progress in silicon micromechanical electrical system (MEMS). A MEMS thermal conduction sensor allows a high degree of integration and miniaturization. Power consumption, response time and production costs of thermal conduction sensors can thus be dramatically reduced.
Ideally, the MEMS thermal conduction sensor consists of a heated resistor, a heat sink and a gas cavity. The heated resistor is located on a thin film bridge and thus thermally insulated against a supporting substrate. The gas cavity is located between the heat source and the heat sink. When a MEMS thermal conduction sensor is in operation, heat transfers from the heat source to the heat sink. Any change in the gas concentration, and thus the thermal conductivity of the gas cavity, results in a change in the bridge temperature. This change in temperature is measured with a temperature sensor, which is also located on the thin film bridge and is thermally linked very closely to the heated resistor.
Gerhard Pollak-Diener and E. Obermeier presented a micromachined thermal sensor which is suitable for the analysis of binary and ternary gas mixtures. Both the heat source and heat sink of the sensor are made of silicon. A first silicon wafer is etched in hot potassium hydroxide in order to produce a thin film bridge for carrying a heated resistor. A second silicon wafer is etched to form a cavity and used as a heat sink. The second silicon wafer is mounted above the first silicon wafer. The first silicon wafer is supported on the back side by an insulated wafer. (Heat-conduction micro-sensor based on silicon technology for the analysis of two- and three-component gas mixtures, Sensors and Actuators B, 13-14 (1993) 345-347).
Arndt Michael and Lorenz Gerd revealed a thermal conduction sensor including a thermally insulated diaphragm formed by a recess in a base plate exhibiting poor thermal conductivity. On one or both sides, the diaphragm is covered by a porous cover plate permitting gas exchange by diffusion, a cavity being left open between the diaphragm and the porous cover plate. (U.S. Pat. No. 7,452,126, Arndt, et al., Nov. 18, 2008).
Several disadvantages can be found in the above first design. In the sensor fabrication process two silicon wafers are first processed individually. Then the wafers are bonded together through a wafer-level bonding process. Finally, the stack of the bonded wafer is mounted on the surface of a insulate wafer. Such complicated and cumbersome process is far from efficient and economical.
In the above second design the fabrication process also consists of wafer-level bonding. Furthermore, the silicon wafer is required to bond to a particular silicon carbine plate or an aluminum oxide plate instead of a commonly used silicon wafer or a glass wafer.
In 2002 year, Isolde Simon and Michael Arndt reported a micromachined conductivity sensor for hydrogen detection. A “hot” element of the sensor is realized as a platinum heat source structure on a thin film silicon-nitride dielectric bridge. A “cold” element is formed by the bulk silicon surrounding the bridge and by the gas surrounding the sensor. (Conductivity sensor for the detection of hydrogen in automotive application, Sensors and Actuators, A97-98 (2002) 104-108)
In 2008 year, S. Udina et al described a thermal conduction sensor for natural gas analysis. The sensor structure consists of a thin film bridge defined on a silicon chip. The bridge is a multilayer sandwich structure of silicon dioxide/silicon nitride and used as a hotplate. A polysilicon heat source is located in the hotplate and a boron-doped silicon island is located right below as a thermal spreader for better temperature homogeneity across the hotplate. The backside of the die is attached to a metal casing (TO-8) acting as a heat sink to keep the substrate temperature approximately constant. The TO-8 casing presents a drilled hole in the area right below the bridge, which improves gas exchange with the surrounding atmosphere. (A micromachined thermoelectric sensor for natural gas analysis: Thermal model and experimental results, Sensors and Actuators B 134 (2008) 551-558).
In the above two design the heat generated by heat source flows through the surrounding gas instead of a gas cavity. In an open space it is impossible to limit the heat transfer to be conduction, because the Rayleigh number may be much higher than a critical number for natural convection to occur. The temperature behavior of the heater depends on both conduction and natural convection, experimental data exploitation will be quite tricky and difficult to analyze.